There is interest in forming relatively light-weight aluminum alloy and magnesium alloy sheet materials into, for example, automotive vehicle body panels. Such panels may be formed from initially flat, sheet metal blanks having nominal dimensions of, e.g., about 1000 mm×1500 mm×1−3 mm. So far, automotive manufacturing engineers have had more experience in forming body panels from aluminum sheet alloys, although magnesium alloys are hot formable at about the same temperature ranges as aluminum alloys and offer further reductions in weight.
The difficulty in forming large, thin panels depends largely on the complexity of the shape of the panel, the severity of the deformation required to be introduced into a sheet metal blank. Some panel shapes, like engine compartment hoods, can often be formed by stamping aluminum alloy sheet blanks between complementary, facing forming dies without preheating the workpieces. One or both of the dies have convex (ram) surfaces that stretch the sheet metal into and against a concave surface on the facing tool. The stamping is carried out at the ambient temperature of the manufacturing site. Other, more complex panel shapes have required that the workpieces be preheated for hot stamping or hot blow forming. Aluminum vehicle lift gates and door panels often require high forming temperatures to deform the sheet material into a decorative and functional panel shape.
Hot blow forming of magnesium or aluminum sheet metal typically involves heating of the sheet to approximately 500° C. in a preheat furnace, robotically transferring that sheet to a position between facing dies which are also heated to approximately that same temperature, clamping the sheet between die halves to establish a gas-tight seal, and then applying gas pressure to one side of the sheet to blow it into a facing die cavity to form the desired shape. Later, the gas pressure is released, the die is opened, and the formed panel is removed and allowed to cool. Alternatively, in some cases, instead of using a preheat furnace, the sheet may be heated by the hot die. In either case, the sheet is typically heated to approximately 500° C., and then held at that temperature for a short time to assure uniform temperature prior to application of the forming pressure. The workpiece typically (if not already fully annealed) undergoes static recrystallization before deformation, and it is the recrystallized grain structure that experiences the deformation. This practice is successfully used with aluminum alloys of suitable composition and thermomechanical history.
In forming by hot stamping, the aluminum or magnesium alloy sheet material is usually preheated to a temperature below about 350° C. and stamped between heated, complementary forming dies carried on opposing press platens and maintained at a specified forming temperature. Again, if the workpiece is not already fully annealed and the preheat temperature is above the static recrystallization temperature, the workpiece will undergo static recrystallization before any deformation. Upon press closure, the heated sheet is contacted by at least one die surface which rams and stretches the sheet against a facing surface. As in hot blow forming, the sheet workpiece and the hot stamping tools are at a specified hot stamping temperature before deformation of the workpiece begins.
These hot forming practices are well developed for aluminum sheet alloys and the fully preheated workpieces are formed readily into body panels of complex shape. But such hot forming of magnesium sheet alloys has generally been slower and more easily applied to the forming of panels with lower shape complexity.